Somatostatin is a cyclic tetradecapeptide which inhibits the secretion of pituitary growth hormone. Several analogs of somatostatin have been previously described. Verber et al., Nature, 292, 55, (1981) has reported the synthesis and the activity of a cyclic hexapeptide analog obtained by replacing 9 of the 14 aminoacids of somatostatin with a single proline. Additionally, the same group has reported hexapeptide derivatives of high potency, Life Sciences, 34, 1371 (1984). Bauer, et al., in "Peptides" 1982, p. 583, Ed. K. Blaha, P. Malon--1983 by Walter de Gruyter & Co., Berlin, New York, describe the synthesis and activity of octapeptide analogs. (cf. also Life Sciences, 31, 1133, L982)).
In general, the abbreviations used herein for designating the amino acids and the protective groups are based on recommendations of the IUPAC-IUB Commission of Biochemical Nemenclature, see Biochemistry, 11, 1726-1732 (1972). For instance, Abu, Ala, Gly, Cys, Lys, Asn, Asp, Phe, Trp, L-Trp, D-Trp, DL-Trp, D-5-Br-Trp, L-5-F-Trp, Thr and Ser represent the "residues" of .alpha.-aminobutyric acid, L-alanine, glycine, L-cysteine, L-lysine, L-asparagine, L-aspartic acid, L-phenylalanine, L-tryptophan, L-tryptophan, D-tryptophan, DL-tryptophan, D-5-bromotryptophan, L-5-fluorotryptophan, L-threonine and L-serine, respectively. The term "residue" refers to a radical derived from the corresponding alpha-amino acid by eliminating the hydroxyl of the carboxyl group and one hydrogen of the alpha-amino group.